1. Field of the Invention
This invention relates generally to drilling of oilfield wellbores and more particularly to apparatus and method for processing (separation, resizing and/or disposition) at the sea floor at least a portion of solids returning with the drilling fluid to the sea floor wellhead during drilling of subsea wellbores.
2. Description of Related Art
Oilfield wellbores or boreholes are drilled by rotating a drill bit attached to the bottom of a drill string. The drill bit is rotated by rotating the entire drill string from the surface and/or by a drilling motor (also referred to in the oil and gas industry as the xe2x80x9cmud motorxe2x80x9d) disposed in a bottomhole assembly attached to the drill bit. In either case, a drilling fluid, which is usually a mixture of water or oil and various additives (commonly referred to as the xe2x80x9cmudxe2x80x9d) is supplied under pressure from a source thereof at the surface into the drill string tubing, which may be a jointed pipe or coiled tubing.
In drilling subsea wellbores, the drilling fluid passes through the tubing, bottomhole assembly and drilling motor (when used) and discharges at the drill bit bottom. The drilling fluid discharging at the wellbore bottom and then returning to the wellhead at the sea floor via the annular space (the xe2x80x9cannulusxe2x80x9d) between the drill string and the wellbore wall. The rock disintegrated by the drill bit rotation (commonly referred to as the xe2x80x9ccuttingsxe2x80x9d or the xe2x80x9cdrill cuttingsxe2x80x9d) is carried to the subsea wellhead by the returning drilling fluid via the annulus. Additionally, solids may enter into the returning drilling fluid due to caving of the rock along the drilled wellbore. Solids may also be present in the form of metal cuttings due to cutting of holes in metallic pipes to form junctions for drilling lateral wellbores or in the form of chunks of cement dislodged from completed or partially completed sections of the wellbore. The returning drilling fluid carrying the above-described solids is sometimes referred to herein as the xe2x80x9creturn fluidxe2x80x9d or xe2x80x9cwellstream.xe2x80x9d
During drilling of certain types of subsea wellbores, such as dual gradient type, solids in the fluid returning to the wellhead at the sea floor must be properly managed. In certain subsea applications, the return fluid is pumped to the surface by pumps at the sea floor. These pumps can allow passage of solids of up to a certain size and hardness without being damaged. The solids, however, can vary greatly in size and hardness, with some solids being greater in size than the pump specifications. Unless the oversized solids are first removed or resized, they can plug or damage the pumps. Replacing pumps in deep sea drilling operations can be very expensive.
Thus, the subsea pumps must be protected from plugging or damage caused by the impact of the drilling fluid solids on the internal parts of the pumps. Particle impact, depending upon the size, hardness, and density of the solids, can gradually erode the pumps, cause catastrophic failures, or plug the pumps.
The present invention addresses the above-noted problems and provides apparatus and methods for processing the return fluid including separating solids, resizing solids and transporting the fluid to the surface without plugging or damaging the subsea pumps.
The present invention provides apparatus and methods for processing of drilling fluid returning to the wellhead in subsea drilling operations. In one aspect, the invention provides apparatus and methods for controlling the particle size of the solid mass present in the circulating drilling fluid returning to the subsea wellhead during drilling of a subsea wellbore. The system includes a separator at the sea floor adjacent the wellhead, which separates solids above a predetermined size from the return fluid. The subsea separator may be a mechanical separator, a hydrocyclone-type separator or any other type of separator judged suitable for the task. The return fluid from the separator enters into one or more subsea pumps, which pump the fluid to the surface. A crusher or pulverizer, either integrated in the separator or as a separate unit, receives the separated solids and reduces them to relatively small-sized particles. The small particles are then pumped to the surface by subsea pump(s) which may be the same pumps utilized for pumping the return fluid to the surface or separate subsea pumps. Alternatively, the separated solids may be collected from the separator into a container. The container is then transported to the surface by a suitable method. Alternatively, filtered drilling fluid may be used to lift the collected solids to the surface. In an alternative system, the return fluid may be passed directly to a crusher that reduces the particle size of the larger solids. The fluid and the small solids are then pumped to the surface. The crusher and the pump may be integrated into a common unit or may be separate serially arranged units. Solids in the wellstream reaching the surface are filtered or removed by conventional methods. The filtered fluid is conditioned to obtain the desired drilling fluid properties. This conditioned fluid is pumped back into the wellbore as the drilling fluid.
Examples of the more important features of the invention thus have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto.